PHYSIOGRAPHY OF BISHOP CONGLOMERATE 627 



or six feet in height. All things point toward the conclusion that 

 this dissection is still in progress. 



A slight increase in precipitation is thought to be the cause of 

 the dissection. What but a change in the amount of rainfall 

 would account for the building of desert fans across a small stream- 

 valley and their subsequent dissection by the stream? 



Confirmative evidence of recent increased precipitation is 

 furnished by a belt of sand dunes about forty miles north of Rock 

 Springs. This belt of dunes begins in the broad plain of the valley 

 of Big Sandy Creek near its junction with Green River, and extends 

 eastward through a low gap in the scarp west of the Leucite Hills, 



Fig. 10. — Looking across Salt Wells Creek toward Chimney Rock, showing the 

 flat "valley-fill" in which the stream is developing a series of terraces. Three dis- 

 tinct terraces are found at this point. The view gives a good idea of the general 

 character of the flats formed during the latest period of aggradation. Chimney 

 Rock is nearly one-half mile distant, and the flat is here over one-quarter mile in 

 width. (Photo, by B. L. Johnson.) 



between two of the larger of these, Steamboat and North Table, 

 and thence eastward across the plain of the Red Desert for many 

 miles. The belt of dune sand varies from one or two to five miles 

 in width. All along the dune strip the vegetation is encroaching 

 on the dunes. In crossing from one side to the other, one passes 

 first an area from one-fourth to one-half mile in width of stagnant, 

 sage-covered dunes, then a belt of active dunes with almost no 

 vegetation, and finally on the other side another belt of sage- 

 covered dunes. West of the pass, on the plains of the Green River 

 valley, the strip of active dunes rapidly narrows to a wedge which 

 pinches out about four miles west of the pass. Here, and farther 



